1. Field of the Invention
This invention relates to electrical load protection devices. More particularly, this invention relates to a three phase electrical load protection device which monitors the three phase supply mains connected to a three phase electrical apparatus to disconnect the electrical apparatus from the supply mains when the supply voltages exceed or fall below adjustable predefined limits.
2. Description of the Background Art
Presently, there exist many types of electrical load protection devices designed to protect an electrical apparatus from overvoltage and undervoltage conditions. More particularly, U.S. Pat. No. 3,657,603 discloses an electrical load protection device connected to a switching relay which disconnects the supply voltage to the electrical apparatus when the line voltage is either below a bottom limit for a definite length of time or above a top limit. The device then automatically reconnects the line voltage to the electrical apparatus a predetermined time after the supply voltage returns to a value between the predefined top and bottom limits. U.S. Pat. No. 4,584,623 discloses another electrical load protection device which provides for a selection of a nominal line voltage which then determines the actual minimum and maximum pressure threshold voltages. This device also includes a delay circuit for delaying the reconnection of supply voltage to the electrical load for a period of time after the supply voltage returns to a normal level.
Related voltage and current protection devices, particularly adaptable for d-c operation, are disclosed in U.S. Pat. Nos. 3,590,325, 3,341,748, 3,679,964 and 3,800,198.
Each of the load protection devices disclosed in the above referenced patents are operable in conjunction with only single phase supply voltages. Such devices are not capable of simultaneously monitoring the voltage levels of a three phase supply, nor are such devices capable of determining when such phases have been improperly connected to the electrical load.
Therefore, it is an object of this invention to provide an apparatus which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the electrical load protection art.
Another object of this invention is to provide a three phase electrical load protection device for monitoring three phase supply mains connected to an electrical load apparatus which disconnects the electrical load from the supply mains if any phase of the mains fall outside predefined over or undervoltages.
Another object of this invention is to provide a three phase electrical load protection device which monitors the phase rotation of three phase supply mains to prevent voltage from being supplied to the electrical load when the direction of such phase rotation is improper.
Another object of this invention is to provide a three phase electrical load protection device including means for automatically restoring the three phase supply mains a different predetermined delay after disconnecting and a predetermined time after the mains are restored to prevent the electrical load from being cycled too rapidly.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.